Abstract: Investigating the reproductive consequences of consanguinity in historical Krummhörn, we show social-group specific associations with correlates for fertility and breeding success like the risk of permanent grandchildlessness. While the proportion of mothers, for which not any birth of a grandchild has been documented at all is much lower among socioeconomically privileged farmers, they will even have more often have at least one grandchild born in this population if married consanguineously. However, opposite effects are observed in families without known landholdings for which consanguinity is associated with increased risk of grandchildlessness. We interpret our findings in regard to local resource competition, but also discuss limitations of our study due to data constraints.
https://t.co/G5MYfz8NxV Sadly, I realized too late that @EvolHumBehav omitted to include important metadata (like #rstats code) in our supplement. Pls find the @Rstudio project (incl preprint) at https://t.co/aQKD7wBaAG
— Johannes Johow (@j0h0w) January 10, 2019
Sample selection: Marriages between 1720 and 1830
Starting with the sample from https://gitlab.com/johannes.johow/consanguinity we find the extinction risk (i.e. the proportion of couples not having ever any grandchild born in the study population) peaks for couples married after 1830 (Fig. 1). In the following, only couples married before 1830 have been included in analysis. Note also, that - on average - extinction risk is higher and number of grandchildren ever born is lower among families without known landholdings (‘unknown/landless’) compared to landholding families (‘landowner’).
Total fertility and breeding success
Comparing the number of children ever born of ‘non-consanguineous’ couples (i.e. F < 0.0156) to ‘consanguineous’ couples (i.e. F >= 0.0156), one finds opposite effects depending on land-ownership (Fig. 2): While number of children ever born is estimated significantly higher for ‘consanguineous’ couples in landowning families, estimates do not significantly differ (but seem to increase rather regarding parametric means) in the case of families being landless or with unknown amount of landholdings. However, consanguineous couples both among families with or without knonw landholdings do on average not have more grandchildren ever born (but rather fewer - although this estimated decrease is not shown to be significant).
Total fertility according to SES and consanguinity
Modeling grandchildlessness in a binomial model
The previously used sample has been restricted to those marriages contracted before 1830 in order to ensure that the birth of grandchildren can be sufficiently traced. The binary outcome of grandchildness is modeled in a mixed-intercept logistic regression allowing for baseline changes according to marriage periods and including consanguinity (non-consanguineous (F < 0.0156)
vs. consanguineous (F >= 0.0156)
) and landholdership (unknown/landless
vs. landholding
) as fixed effects interaction.
gc_extinct | |
(F > = 0.0156) | 1.056^{**} |
(0.332) | |
with_grasenlandholding | -0.573^{***} |
(0.130) | |
(F > = 0.0156):with_grasenlandholding | -2.537^{*} |
(1.059) | |
Constant | -1.855^{***} |
(0.102) | |
N | 5,396 |
Log Likelihood | -1,863.713 |
AIC | 3,737.426 |
BIC | 3,770.393 |
^{}p < .05; ^{}p < .01; ^{}p < .001 |